Many existing printing devices are bi-level devices that cannot readily reproduce continuous tone images. Thus, a continuous tone image is approximated by first defining a halftone grid, known as a screen. The screen is essentially an array of regions known as halftone cells. Each halftone cell typically has a fixed size, and is defined by a matrix of addressable pixels that can be selectively turned "on" in a digital, "bi-level" manner to form various patterns. The human eye integrates the array of halftone cells to form a visual perception of a continuous tone image. A gray value is assigned to each halftone cell within the screen in order to represent the gray value of the corresponding areas of the continuous tone image. By activating a percentage of the pixels contained within each halftone cell, the cell simulates a shade of gray which closely approximates the respective area of the continuous tone image. For example, in order to approximate a lighter area of the image, a smaller percentage of pixels, such as 10%, of the halftone cell will be activated. To simulate a darker image region, a higher percentage of the pixels will be activated. These techniques are well known in the art.
A conventional printing device produces halftone images by forming halftone dots on a medium at locations corresponding to each pixel that has been turned "on" in the respective halftone cell. The process of forming the halftone dot is particular to the type of printing device. For example, the spots may be formed by depositing ink or toner on a printing substrate at locations corresponding to the activated points. Alternatively, spots may be formed on a photographic film or a thermographic film by exposure to a radiation or thermal energy, respectively. Other printing devices employ processes such as dye sublimation or thermal mass transfer as are known in the art.
Many printing devices reproduce an original color image by separating the image into color components such as yellow, cyan, magenta and black. The color components are independently formed on a respective medium according to the halftone process described above. For example, in offset printing a printing plate is created for each color component and the color image is reproduced by overprinting colored inks.
Dot gain is a well known problem associated with halftone systems and refers to an apparent change in size of a printed halftone dot from its target size. This phenomenon is caused by many factors such as a tendency of ink to spread or variations in film characteristics. For example, when 50% of the dots within a halftone cell are exposed, the resulting dark area may cover more than or less than 50% of the total area defined by the halftone cell. Typically, this is due to nonlinear effects in the imaging system, film, media or processing system. Because 0% and 100% are usually achievable, a non-linear relationship may exist between the target dot area and the resultant dot area.
A subset of conventional printing devices, referred to as imagesetters, consist of a front-end raster image processor (RIP) and a recording device for producing the image on film or paper. Manual calibration techniques are well known in the industry as a means for calibrating a halftone imagesetter so as to compensate for dot gain. Typically an operator of a printing device uses a densitometer to detect dot gain. A densitometer is an instrument that measures the perceived optical density of the reproduced image. A densitometer typically consists of a light emitting component for illuminating the reproduced image and a photodetector for measuring light reflected from the image. Alternatively, the photodetector measures light transmitted through the reproduced image. The darker the image the more light it absorbs and the higher the density reading from the densitometer. During the calibration process a grayscale test pattern is printed which includes a series of halftone image regions. Each image region has a different predetermined dot area. For example a series of image region is usually printed such that the dot areas range from 2% to 100%. The operator manually measures the density of each image region with a standard densitometer. From these measurements, a "transfer function" is created to map any subsequently requested dot area to a dot area which produces the correct visual density.
Conventional calibration methods operate at either the application level or at the RIP level. Application level methods send a transfer function with each print job. On the other hand, RIP-based compensation techniques require the RIP to store transfer functions. The operator selects the correct transfer function based on current operating conditions. If the operating conditions change, such as the use of a new media type, the operator generates a grayscale test pattern, manually measures the densities with a densitometer, generates a transfer function and designates the new function for current use. If no major system change occurs, the functions may be used for an extended period such as several weeks.
Another RIP-based calibration technique sequentially changes software input variables such as resolution, frequency and medium (film/paper). A new test pattern is printed for each combination. The operator manually measures each test pattern with a densitometer and creates a plurality of transfer functions. The RIP selects the correct transfer function based on the current print job.
The above-described calibration approaches require the operator to determine when calibration is appropriate and therefore require substantial operator interaction. As such, they fail to adjust for drifts in overall system transfer function. Additionally, they fail to adequately account for the sensitometric response of the film to different levels of exposure and for factors introduced by lot-to-lot variations of similar film types.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a method and device for calibrating a halftone printing system without requiring operator intervention, thereby adapting to drifts in overall system performance including variations in media characteristics and media development parameters. Furthermore, there is a need in the art for a calibration device that minimizes imaging errors due to dot gain.